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A335902 Composite numbers c such that phi(c)/phi(mind(c)) mod phi(c)/phi(maxd(c)) <> 0, where phi is the Euler function, mind(c) is the smallest nontrivial divisor of c, maxd(c) is the largest nontrivial divisor of c. 3
35, 55, 77, 95, 115, 119, 143, 155, 161, 187, 203, 209, 215, 221, 235, 245, 247, 253, 287, 295, 299, 319, 323, 329, 335, 355, 371, 377, 391, 395, 403, 407, 413, 415, 437, 473, 493, 497, 515, 517, 527, 533, 535, 551, 559, 581, 583, 589, 605, 611, 623, 629, 635, 649 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This equivalence criterion splits a set of composite numbers into two classes and can be used to count certain combinatorial objects.

LINKS

Table of n, a(n) for n=1..54.

PROG

(MATLAB)

n=500; % gives all terms of the sequence not exceeding n

A=[];

for i=1:n

    dn=divisors(i);

    if size(dn, 2)>2 && mod (totient(i)/totient(dn(2)), totient(i)/totient(dn(end-1)))~=0

        A=[A i];

    end

end

function [res] = totient(n)

res=0;

    for i=1:n

        if gcd(i, n)==1

            res=res+1;

        end

    end

end

(PARI) isok(c) = if ((c>1) && !isprime(c), my(t=eulerphi(c), d=divisors(c)); ((t/eulerphi(d[2])) % (t/eulerphi(d[#d-1]))) != 0); \\ Michel Marcus, Dec 28 2020

CROSSREFS

Cf. A000010, A002808, A340058.

Sequence in context: A318572 A171082 A340269 * A121707 A267999 A319386

Adjacent sequences:  A335899 A335900 A335901 * A335903 A335904 A335905

KEYWORD

nonn

AUTHOR

Maxim Karimov, Dec 28 2020

STATUS

approved

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Last modified April 16 18:53 EDT 2021. Contains 343050 sequences. (Running on oeis4.)